Historically, unfractionated heparin has been used in the treatment and prevention of thromboembolic disorders. However, due to complications associated with heparin (such as bleeding), low molecular weight heparins (LMWHs) have been developed and are being used in place of unfractionated heparin. LMWHs have proven effective as antithrombotic agents; however, the effects of LMWHs cannot be readily reversed while unfractionated heparin can be reversed by protamine sulfate. The overall goal of this proposal is to develop an antithrombin (ATIII)-specific RNA aptamer that will combine the safely and efficacy of LMWH with the reversibility of unfractionated heparin. To accomplish this goal, an oligonucleotide antidote for this aptamer that will be used to rapidly reverse the anticoagulant properties of the aptamer will be developed. Proof of this concept is established by the development of an RNA aptamer and RNA aptamer antidote directed against coagulation factor Xa. In collaboration with this group, an antidote-controlled ATIII-specific RNA aptamer will be developed. This aptamer will mimic the action of LMWH by binding to the D-helix of ATIII and enhance ATIII inhibition of factor Xa. This aptamer/antidote pair will have advantages over LMWHs because it will not only be able to control thrombosis similar to LMWHs but the availability of an antidote will allow for better therapeutic regulation and intervention. The experiments proposed in this proposal will characterize this family of antithrombin-specific RNA aptamers using both in vitro and in vivo approaches.